A while back, I rashly accepted a job that involved providing English equivalents for a list of Spanish fish names. It was one of the least profitable pieces of work I have ever done, given that it seemed to take about 10 minutes per word (translating poetry in the original metre would have been quicker, and more fun), but I did discover the joys of Fishbase (www.fishbase.org), which, though it doesn't solve every problem, is a really essential tool for this purpose: a vast database which gives full details and common names in many languages and different countries.

I will not bore you with all the ramifications of this apparently innocent name "amarillo". Suffice it to say that it is very difficult to pin down what it refers to. The obvious place to start is to see what sort of fish people are catching in the Sierra de la Macarena National Park. Here, Alfonso's source is very valuable. It is specifically about fishing in this park. It has a picture of "Amarillos capturados por pescadores de la región", adding that "pueden llegar a pesar hasta 200lb" (a very important point: they're big fish), and it refers to "los pescadores de la región luchando contra los grandes bagres amarillos".
http://pescandoencolombia.blogspot.com.ar/2012/03/macarena-f...

So then we are talking about the "bagre amarillo". This is corroborated by another blog piece, on "Pesca de Bagres en el Orinoco Colombiano", with photos of a proud angler holding a monstrous greenish-yellow fish:
http://pescadecolombia.blogspot.com.es/2013/01/pesca-de-bagr...

The park includes part of the the Orinoco basin region of Colombia (Orinoquía):
https://es.wikipedia.org/wiki/Sierra_de_la_Macarena

Other sources confirm that "bagres amarillos" are found here.

So what is a bagre amarillo? Here the trouble really starts, because the name is found associated with a number of different species, even within Colombia. However, most of them can be ruled out for one reason or another. Danik is right that it seems to be some sort of catfish. But it can't be the yellow bullhead (Ameiurus natalis), which is confined to North America.

Fishbase gives two possibilities for this name, but neither of them can be right here:
(a) Pimelodus maculatus (no English common name), known as bagre amarillo in Argentina and Uruguay and also as amarillo in Argentina. But it's not found in Colombia, it looks nothing like the ones in the pictures, and it's small (max. published weight 2.4 kg).
http://www.fishbase.org/Summary/SpeciesSummary.php?ID=23451&...
(b) Sciades parkeri (gillbacker sea catfish), which is found in Colombia, and is big (max. length 190 cm., max. published weight 50 kg), but is an estuary fish and would certainly not be found in this park.
http://fishbase.mnhn.fr/Summary/SpeciesSummary.php?ID=952&AT...

Scouring Colombian sources for alternative species called "bagre amarillo" throws up a lot of false leads, but also one that I think provides the right answer, among the publications of a Colombian marine biologist:

"Primera reproducción en cautiverio de Bagre Amarillo Paulicea lüetkeni. Revista ACUIORIENTE V4, 1999"
http://web.unillanos.edu.co/docus/hv_javier_alvarez.pdf

Paulicea lüetkeni is an alternative name for Zungaro zungaro, the gilded catfish; Distribution: South America, Amazon and Orinoco basins; "Specimens measuring 130 cm and weighing 50 kg are not rare".
http://fishbase.mnhn.fr/Summary/SpeciesSummary.php?ID=47756&...

Here's a proud Brazilian angler holding one:
http://www.pousadarioxingu.com.br/peixes/index_ing.html#jau

Its range is fairly wide but includes Colombia.
https://es.wikipedia.org/wiki/Zungaro

I think this must be it.


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Note added at 5 hrs (2015-09-17 06:28:20 GMT)
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Sorry Adolfo; I mistakenly called you Alfonso.